BinarySearch array of objects by ID

Question

Good day!

I have a List of ValueObj:

class ValueObj
{
   int ID;
   float value;
}

How to get binary search objects by id? (List tempValues)

I make ValueComparer class,but dont know am i right?

class ValueComparer<ValueObj>
{
   public int Compare(ValueObjx, ValueObjy)
   {
       if (x == y) return 0;
       if (x == null) return -1;
       if (y == null) return 1;

       return -1; ///???
   }
}

I need to sort List by ID. Like that?:

tempValues.Sort(new ValueComparer());

And how to use BinarySearch?

Hamid Pourjam · Accepted Answer · 2014-11-06 19:18:58Z

first of all you should make your class like this. your fields were not public and you can not access them, also public fields are not good so you should change them to property

    class ValueObj
    {      
        public int ID { get; set; }
        public float value { get; set; };
    }

and your comparer like this

class ValueComparer : IComparable<ValueObj>
{
  public int Compare(ValueObj x, ValueObj y)
  {
      if (ReferenceEquals(x, y)) return 0;
      if (x == null) return -1;
      if (y == null) return 1;

      return x.ID == y.ID ? 0 :
               x.ID > y.ID ? 1 : -1;
  }
}

then you have a list like

var tempValues = new List<ValueObj>();
//many items are added here

you should always sort your list before performing a binary serach

 //this does not modify the tempValues and generate a new sorted list
 var sortedList = tempValues.OrderBy(x => x.ID).ToList();

or you can sort tempValues directly

//tempValues is modified in this method and order of items get changed
tempValues.Sort(new ValueComparer<ValueObj>());

now you want to find index of specific ValueObj

var index = sortedList.BinarySearch(specificValueObj, new ValueComparer<ValueObj>());

or if you have used second method of sorting

var index = tempValues.BinarySearch(specificValueObj, new ValueComparer<ValueObj>());

Steven Magana-Zook · Accepted Answer · 2014-11-06 21:06:42Z

0

The List class in C# has a BinarySearch method you can use with your Comparable.

Your type:

class ValueObj
{
    public int ID{ get; set;}
    public float value { get; set;}
}

Your comparison class (do not forget to implement the correct interface!):

class ValueObjIDComparer : IComparable<ValueObj>
 {

    public int Compare(ValueObj x, ValueObj y)
    {
        if (x == null) return -1;
        if (y == null) return 1;

        if (x.ID == y.ID) return 0;            

        return x.ID > y.ID ? 1 : -1;
    }
 }

Executing a binary search:

List<ValueObj> myList = new List<ValueObj>();
myList.Add(new ValueObj(){ID=1});
myList.Add(new ValueObj(){ID=2});
// ...

int idToFind = 2;
myList.Sort(new ValueObjIDComparer());
int indexOfItem = myList.BinarySearch(idToFind, new ValueObjIDComparer());

There are many more operations you can perform on lists. See the documentation here.

edited Nov 6, 2014 at 21:06

answered Nov 6, 2014 at 18:33

Steven Magana-Zook

2,77828 silver badges42 bronze badges

2 Comments

Steven Magana-Zook Over a year ago

@HamidP You're right, thanks for pointing that out. For some reason I thought the method would do that for you. Answer has been updated.

Rene Over a year ago

You're searching on idToFind. Wich is an int. But the BinarySearch method needs an ValueObj as first parameter.

nim · Accepted Answer · 2014-11-06 18:18:59Z

0

If you want to sort by ID, you could simply compare the IDs in Compare method:

return x.ID > y.ID;

answered Nov 6, 2014 at 18:18

nim

3823 silver badges14 bronze badges

1 Comment

Steven Magana-Zook Over a year ago

That return statement returns a bool not an int.

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